• Additive vs. Subtractive
There are two different ways to mix colors.
• Additive vs. Subtractive
What do you get if you use a prism to combine
all wavelengths of light?
• Additive vs. Subtractive
What do you get if you use a prism to combine
all wavelengths of light?
• Additive vs. Subtractive
What do you get if you mix a bunch of paint?
• Additive vs. Subtractive
What do you get if you mix a bunch of paint?
• Additive vs. Subtractive
• Additive mixing is most intuitive:
ADD wavelengths:
red+green = yellow
red+blue = magenta
blue+green = cyan
red+green+blue=white
Exploring Additive Mixing
• use color sliders to adjust R,G,B values
• What color can only exist as a metamer
(an additive mixture of wavelengths)?
In other words, what color cannot be
made with a single wavelength?
• What color can only exist as a metamer
(an additive mixture of wavelengths)?
In other words, what color cannot be
made with a single wavelength?
Magenta
Think about why!
• Subtractive mixing is much less intuitive
(but much more common)
• Subtractive mixing happens when we
mix pigments (paint) together
• Different pigments subtract different
wavelengths:
– red subtracts all but red, blue all but blue,
green subtracts blue and red, etc…
• Example: blue + yellow = green
Technically it’s called “cyan”
Amount of filtering
• The result of a mixture depends on what
wavelengths don’t get absorbed by the
two pigments
blue
green yellow
wavelength
red
Amount of filtering
• Both yellow and blue pigments reflect a
bit of green
blue
green yellow
wavelength
red
• Subtractive mixing is commonly used in
color printers
Exploring Subtractive Mixing
• use color sliders to adjust C, M, Y
values
• Everything you’ve learned so far is
wrong.
• Everything you’ve learned so far is
wrong.
• Well, not really wrong, just far from
complete.
What is color for?
• What is color vision used for?
What is color for?
• What is color vision used for?
– Identification - what is this thing?
– Discrimination - what other things is this
like?
– Communication - indicates this thing to
others
What is color for?
• What is color vision used for?
– Identification - what is this thing?
– Discrimination - what other things is this
like?
– Communication - indicates this thing to
others
• But in each case color refers not to the
illuminating light, but to the surface of
What is color for?
• What is color vision used for?
– Identification - what is this thing?
– Discrimination - what other things is this
like?
– Communication - indicates this thing to
others
• But in each case color refers not to the
illuminating light, but to the surface of
Does the color of an object remain constant
the object
itself conditions?
under
different lighting
What Newton Found (and
everyone believed)
• White light can be split into all wavelengths by
a prism
What Newton Found (and
everyone believed)
• White light can be split into all wavelengths by
a prism
• According to previous theories: two
wavelengths combine to yield intermediate
color and no others
Red + Green = YELLOW
What twist did Land do to this
paradigm that confounds the
conventional understanding of
color mixing?
What Land found:
• Two bands (colors) of the spectrum
recombine to produce all the possible
colors
– provided the appropriate relative amount of
each wavelength is projected
transparency slides
How did Land project the
“appropriate” ratio of
wavelengths?
Short- and Long- “record”
• Capture two greyscale images of
the scene using
filters that allow
only the
film
wavelengths you
will project
Camera
“Long” filter
Object
“short” filter
Projector
“Long” filter
“short” filter
Image
What is Land’s interpretation?
How do we perceive color?
Land’s interpretation:
• perception of color is a weighing of the
ratio of shorter and longer wavelengths
Land’s interpretation:
• perception of
color is a
weighing of the
ratio of shorter
and longer
wavelengths
Why would the visual system
have this design?
Why would the visual system
have this design?
• Hint: “Within broad limits, the actual values
of the wavelengths make no difference, nor
does the over-all available brightness of
each”
Color Constancy
• The “color” of objects is independent of
the ambient light
– yellow bananas and green leaves look
yellow and green regardless of whether
they are viewed in direct sunlight or by the
light of a fire
Color Constancy
• Land Mondrian:
– demonstration of color constancy: all the
wavelengths of the colored squares are
shifted by the same amount into the blue
end of the spectrum - your brain ignores
the shift
Color Constancy
• Tricky question:
– why does a window look blue from the
outside when there’s a TV going inside?
Color Constancy
• Tricky question:
– why does a window look blue from the
outside when there’s a TV going inside?
– The wavelengths emitted by a TV are
mostly in the blue end of the spectrum
Color Constancy
• Really Tricky question:
– why doesn’t a TV look blue?
– Color Constancy causes you to perceive
the areas of the screen with the greatest
proportion of long wavelengths as “red”,
the greatest proportion of short
wavelengths as “blue” and everything else
in between.
Next Time
• ATTENTION!